Generally, it is known to protect various metal fasteners and other metal articles against corrosion by electroplating their surfaces with zinc and chromating the electroplated surfaces. Moreover, it is known to cover the chromated surfaces with polymeric materials.
In Palisin, Jr., U.S. Pat. Nos. 3,790,355 and 3,977,839, hydrophobic, thermosetting polymers made from coating compositions containing hexamethoxy methyl melamine are proposed for covering such chromated surfaces. In Labenski et al. U.S. Pat. No. 4,003,760, so-called fluoroplastic materials, such as polytetrafluoroethylene, are proposed for covering such chromated surfaces.
Parkinson et al. U.S. Pat. No. 3,853,606 discloses a coating composition that is described therein as combining properties of increased holding power, corrosion resistance, and lubrication. The coating composition contains two thermoplastic resins, namely a flexible carboxyl or substituted carboxyl-containing resin and rosin-like resin. An organic solvent is required, which is undesirable in an industrial environment because such a solvent produces hazardous vapors and hazardous wastes.
Metal fasteners used in wooden construction present special requirements, for which polymeric materials used to protect metal articles of other types may not be well suited. These fasteners include nails, staples, and truss plates. Typically, the metal of such fasteners is carbon steel.
Desirably, a polymeric material covering a metal fastener used in wood construction should serve not only to protect the fastener against corrosion but also to cause the fastener to exhibit an increased withdrawal force when the fastener has been driven into a wooden workpiece. Withdrawal force, which may be also discussed in terms of holding power, is the force that is required to withdraw such a fastener from a given workpiece.
Furthermore, a polymeric material covering a metal fastener used in wood construction should resist chipping or tearing away, particularly when the fastener is struck by a driving element of a pneumatically powered, combustion-powered, or other rapidly acting fastener-driving tool.
Optimally, a polymeric material covering a metal fastener used in wood construction would serve moreover to cause the fastener to exhibit a decreased penetration force when the fastener is driven into a wooden workpiece. Penetration force is the force required to drive such a fastener into a given workpiece.
However, it is difficult to identify a polymeric material that would not require an organic solvent when applied to such a fastener and that would serve not only to protect the fastener against corrosion, to cause the fastener to exhibit an increased withdrawal force, and to resist chipping or tearing away, but also to cause the fastener to exhibit a reduced penetration force.
Although known coatings for metal articles may be generally satisfactory in many applications, there has been a need, to which this invention is addressed, for improved coatings for metal fasteners, particularly for metal fasteners suitable for wood construction, such as nails, staples, or truss plates.